1. Field of Invention
Embodiments of this invention relate to active materials for electro-optic devices and electro-optic devices that use the materials; and more particularly to conjugated polymers as active layer materials for electro-optic devices, and electro-optic devices that have conjugated polymer active layers.
2. Discussion of Related Art
The contents of all references cited herein, including articles, published patent applications and patents are hereby incorporated by reference.
Organic photovoltaic (OPV) devices are very promising for low-cost, flexible, light-weight, large area energy generation applications (Cheng et al., Chem. Rev., vol. 109, p. 5868, 2009; Coakley et al., Chem. Mater, vol. 16, p. 4533, 2004; Brabec et al., Adv. Funct. Mater., vol. 11, p. 15, 2001). Tremendous work on designing new materials (Boudreault et al., Chem. Mater., vol. 23, p. 456, 2011), device structures (Yu et al., Science, vol. 270, p. 1789, 1995), and processing techniques (Padinger et al., Adv. Funct. Mater., vol. 13, p. 85, 2003; Li et al., Nat. Mater., vol. 4, p. 864, 2005; Peet et al., Nat. Mater., vol. 6, p. 497, 2007) has been carried out to improve the power conversion efficiency (PCE) of OPV devices. So far, polymer solar cells (PSCs) based on conjugated polymers as electron donor materials blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor material have achieved over 7% PCE using a bulk heterojunction (BHJ) device structure (Chen et al., Nat. Photon., vol. 3, p. 649, 2009; Liang et al., Adv. Mater., vol. 22, p. E135, 2010). Nonetheless, most of the materials suffer from the inherent disadvantages of either lacking a broad absorption range, which limits the utilization of the full solar spectrum (Chen et al., Acc. Chem. Res., vol. 42, p. 1709, 2009), or relatively low carrier mobility, which requires the use of thinner films for efficient charge extraction. This reduces the external quantum efficiency (EQE) and lowers the photocurrent (Clarke et al., Chem. Rev. vol. 110, p. 6763, 2010). To utilize solar radiation more effectively, one possible solution is to stack multiple photoactive layers with complementary absorption in series to make a tandem PSC (Kim et al., Science, vol. 317, p. 222, 2007). Typically, such a tandem structure has a front cell with a high bandgap material, an interconnecting layer (ICL), and a rear cell with a low bandgap (LBG) material. Furthermore, the structure enables a reduction of potential loss during the photon-to-electron conversion process, and combines the electrical potential of the individual BHJ cells (Kim et al., Science, vol. 317, p. 222, 2007).
Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combing two or more solar cells with different absorption together. However, for polymer solar cells (PSCs), the performance of tandem devices lags behind of single layer solar cell due to the lack of proper combination of low and high bandgap polymers. So far, most of the research on tandem PSCs has focused on improving the ICL and only a few cases have demonstrated high efficiency (Kim et al., Science, vol. 317, p. 222, 2007; Gilot et al., Adv. Mater., vol. 22, p. E67, 2010; Sista et al., Adv. Mater., vol. 22, p. 380, 2010; Chou et al., Adv. Mater., vol. 23, p. 1282, 2011).
Conjugated polymers are polymers containing electron conjugated units along a main chain, and can be used as active layer materials of some kinds of photo-electric devices, such as polymer light emission devices, polymer solar cells, polymer field effect transistors, etc. As polymer solar cell materials, conjugated polymers should possess some properties, such as high mobility, good harvest of sunlight, easy processibility, and proper molecular energy level. Some conjugated polymers have proven to be good solar cell materials. For example, some derivatives of poly(p-phenylene vinylene), such as MEH-PPV and MDMO-PPV, and some derivatives of poly(3-alky-thiophene), such as P3HT and P3OT, and some conjugated polymers with heterocyclic aromatic rings, such as poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and poly[4,8-bis-substituted-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT), have been successfully used as photo-active layer materials. Although the energy conversion efficiency of the solar cell devices based on these polymers has reached to ˜7%, it was much lower than that of inorganic semiconductor solar cells.
Therefore, there is accordingly a need in the art for conjugated polymers that have good photovoltaic effect. As mentioned above, ideal conjugated polymer materials for polymer solar cells should have high mobility, so main chains of the conjugated polymers should have planar structure, which could be helpful to form π-π stacking structures and facilitate charge transfer between two adjacent main chains; they should have low band gap to provide good harvest of sunlight; they also should have proper molecular energy levels matching with electrodes and electron acceptor materials in polymer solar cell devices. It would be desirable to provide conjugated polymers as photovoltaic materials that possess properties as mentioned above.